In a conventional Package-on-Package (PoP) process, a top package, in which a first device die is bonded, is further bonded to a bottom package through solder balls. The bottom package may also include a second device die bonded therein. The second device die may be on the same side of the bottom package as the solder balls.
Before the bonding of the top package to the bottom package, a molding compound is applied on the bottom package, with the molding compound covering the second device die and the solder balls. Since the solder balls are buried in the molding compound, a laser ablation or drilling is performed to form holes in the molding compound, so that the solder balls are exposed. The top package and the bottom package may then be bonded through the solder balls in the bottom package.
There is significant mismatch between the Coefficients of Thermal Expansion (CTEs) of the materials in the PoP packages. For example, the package substrate and the molding compound have CTEs that are much higher than that of the device dies. Accordingly, in the resulting package, there is a significant warpage.